North Korea is reportedly planning to launch an Earth observation satellite. (AP)

Less than a month after its purported H-bomb test, North Korea has announced it is planning a rocket launch as soon as next week.

Though speculation of a launch had been growing for about a week, experts say that with underground railways, giant tarps and a movable launch pad structure in place North Korea is getting a lot better at hiding its preparations.

North Korea's announcement it is preparing a rocket, which it made by informing international organisations of a February 8-25 launch window, comes after what it claimed was its first H-bomb test on January 6 and statements by American and Japanese officials that they were seeing heightened activity at its main rocket facility.

North Korea typically informs groups such as the International Maritime Organisation (IMO) of a pending launch so that cautions can be issued for shipping in the area. The IMO confirmed it had been informed of the plan on Tuesday.

The announcement ends speculation over whether North Korea was actually preparing a rocket. Though it says the rocket will carry a Kwangmyongsong - or Bright Star - Earth observation satellite, the type of rocket that will be used is not yet clear.

There are indications - including the construction of a new and taller gantry, visible in commercial satellite imagery - that it could be a bigger and better version of the Unha 3 space launch vehicle that lifted off from the Sohae facility in 2012, on the west coast of North Korea.

That would be in line with North Korea's own previous announcements.

The Unha 3 successfully delivered North Korea's first satellite into Earth orbit.

A January 2013 report by Rodong Sinmun, the ruling party newspaper, which has since been deleted from its online edition, quoted a scientist as saying there would be a series of launches of observation and communication satellites culminating with Unha 9, which would carry a lunar orbiter.

A North Korean space agency official told an Associated Press television crew last year that more satellite launches are planned in the years ahead.

Models of the larger and much more formidable-looking Unha 9 rocket have since been displayed at various events in North Korea, including annual flower shows held in honour of national founder Kim Il Sung and his son, Kim Jong Il.

Although there are important differences, the United States and others have strongly criticised such rocket launches because similar technologies can be used in the development of ICBMs, which North Korea is banned from doing under UN restrictions. North Korea says it has the right to maintain a peaceful space programme.

Tightening its punitive squeeze on North Korea, the US treasury on January 17 announced sanctions on 11 individuals and entities involved in Iran's ballistic missile programme, including Iranian officials it said had direct links to North Korea and work being done by the North on "an 80-tonne rocket booster".

It said two of the sanctioned Iranians "have been critical to the development of the 80-tonne rocket booster, and both travelled to Pyongyang during contract negotiations". Iran has, coincidentally, suggested it might also conduct a rocket launch this month.

Whether the booster would be a new first stage for the Unha rockets or something different is not known.

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